Older people suffer from a higher morbidity and mortality from influenza viral lung infections and secondary bacterial pneumonia than younger people. Yet the mechanisms by which aging impairs host defense to respiratory infections are not well understood. Recent human genetic studies have shown that high expression of macrophage migration inhibitory factor (MIF) alleles confer a 50% survival benefit in older individuals with community-acquired pneumonia. In support of this, our preliminary data indicate that aging is associated with reduced MIF within the aging lung in mice. Furthermore, we show that in the lungs, aging and MIF deficiency share several features: increased senescence prior to lung infection, and increased lung damage, and impaired viral clearance after influenza viral lung infection. Importantly, MIF deficiency also worsens outcomes after influenza viral infection followed by S. pneumonia bacterial infection. Here, we will investigate the mechanisms underlying MIF's effects during infectious exacerbations of the aging lung by studying genetically-defined mice infected with two clinically-relevant respiratory models: primary influenza virus, and influenza viral infection followed by S. pneumonia bacterial infection. Aim 1 will investigate mechanisms by which aging and MIF deficiency impair clearance of influenza virus with increased lung damage, with a focus on type I interferon production, an inflammatory mediator that is critical to host defense against viral infection. Aim 2 will investigate the role f MIF in host defense after secondary bacterial infection with aging. Both Aims will test the role of MIF by employing novel MIF transgenic mice and new pharmacological MIF modulators. Therefore, this proposal will yield novel information concerning how the aging lung responds to respiratory pathogens, which could provide novel information to improve therapies for older people who succumb to respiratory infections.